Custom

Custom is a world type that allows users to change the generation of the Overworld, Nether, and End dimensions as well as the ability to create custom dimensions. It is edited using a JSON file that is imported on the world creation screen.

To customize a world by using a JSON file, it needs to be imported before world creation:

The only way to access added dimensions is through commands, such as /execute in <dimension name> run tp @s ~ ~ ~, which teleports the player to the specified dimension (see § JSON format).